Recent improvements in the use of ultrasonic energy to illuminate the internal organs of the human body have included "realtime" imaging, which enables visualization of the motion of the organs, and "gray-scale" imaging, which gives diagnostic information concerning the texture of the soft tissue in the body. More recently, ultrasonic imaging using arrays of transducers with appropriate control electronics has proven successful because of transducer reliability, flexibility and capability for steering and focusing ultrasonic energy, and has improved resolution over a certain depth of field, particularly near the transducer.
Improvement beyond the present state of the art requires larger transducer arrays with more elements, but this requires correspondingly more circuitry. In order to produce such systems economically, innovations must be made in system architecture to permit flexible system operation which makes extensive use of symmetry to minimize dedicated hardware without sacrificing performance, consistent with high resolution imaging.
An example of previous array technology is described in U.S. Pat. No. 4,140,022 granted Feb. 20, 1979 to Samuel H. Maslak and assigned to Hewlett-Packard Company, where extensive use of tap selectors is illustrated. In the there-described system, the number of analog switches required for tap selection depends upon the product of the number of input signals, which is equal to or proportional to the number of active transducer elements, and the number of taps in the summing delay line configuration. In order to improve the performance of such a system, by doubling the number of transducer elements, for example, the number of analog switches must increase by a factor of four to accommodate the longer delay line needed for the corresponding improvement in performance. These analog switches contribute substantially to system cost because they should be low-noise, broad-bandwidth components. It is, therefore, desirable to eliminate the need for tap selection completely. Furthermore, the foregoing patented system provides a sector scan but does not provide for linear scanning.
Another example of the prior art, U.S. Pat. No. 4,005,382 granted on Jan. 25, 1977 to William L. Beaver and assigned to Varian Associates, does avoid tap selection. But the described system is unable to provide the non-monotonic delay profiles required to achieve significant focusing of the ultrasonic wave at angles at or near the normal to the transducer. That patented architecture requires "zero delay" electrical connections among the outputs of adjacent delay elements, which creates severe technological difficulties in a high resolution imaging system with many simultaneously active transducer elements. That patented architecture also lacks the cyclical symmetry needed for a linear scanning format, wherein sequential acoustic lines are activated, each normal to the transducer but displaced by an amount along the transducer array which might be equal to the transducer element-to-element spacing.